This invention is generally directed to wind energy conversion systems and methods.
The classical work is the article by Miles L. Loyd “Crosswind Kite Power” (1979), in which the author disclosed a wind energy harvesting device, comprising a tethered wing, flying crosswind and harvesting wind energy, and transferring harvested energy to a ground based generator via motion of the tether. Crosswind motion of a wing is much more efficient, than downwind motion, allowing the wing to fly many times speed of wind and harvest energy from an area, many times larger than the area of the wing. Unfortunately, velocity of the lengthwise motion of the tether must be well below velocity of the wing. Moreover, Loyd proved mathematically that the optimal downwind speed of the tether is ⅓ of the wind speed, and suggested using this speed in his device. Unfortunately, the tether is subject to very high force, requiring thick tethers and creating very large torque in the ground equipment for useful power.
Other variations of airborne wind energy conversion system with a generator on the ground have been suggested. In some of them, a lift generating airfoil flies downwind, rather than crosswind. In some other systems, a drag based wind capturing device (such as a parachute canopy) is utilized. In some other systems, a combination of aerodynamic lift and drag is used to convert the wind power. And in other systems (such as described in U.S. Pat. No. 6,072,245 by Ockels), multiple airfoils are connected to a single generator. These systems share the drawback of the high force and high torque in the power transfer from wind energy capturing element to the generator.
It should be noted, that the problem of high forces and high torques is common for wind energy conversion devices, even conventional wind mills, and has not been satisfactory solved in hundreds of years.
This invention is directed to solving this problem.